Can you please provide the URI that you are experiencing the issue on?

I researched some more, and here is my understanding.

The element I tried to add 'mousedown' event was inside another element, which was using a jquery library, jquery.panzoom.js. So when i checked that library they had a condition like,

events[ ((!!window.PointerEvent) ? 'pointerdown' : 'mousedown') + ns ] = function() {
          self.transition(true);
};

I think the pointerevent is a new event in chrome 55. So before this version there were no pointerevent, and this library was using 'mousedown' event instead. Since pointerevents are available now, the library binded 'pointerdown' and that broke a child element's mousedown event.
        

Yes Chrome 55 supports pointer events and it appears from github (https://github.com/timmywil/jquery.panzoom) uses pointer events.

Can you please provide us a reproduction URI that we can debug? ie; a jsbin or a jsfiddle would be awesome.

I just reproduced the same on jsbin. 

Please follow the link

https://jsbin.com/fivosay/edit?html,js,output

If you comment out the panzoom initialization, that mousedown event works, otherwise it won't. It works if you edit the panzoom library to always use mousedown instead of pointer events.

The library is preventDefaulting the pointerdown event here: https://github.com/timmywil/jquery.panzoom/blob/master/src/panzoom.js#L969

This causes mousedown event not to be sent as per the pointer event specification.

Before you got the event because it was bubbling (they didn't stop propagation of the event)

So fundamentally the browser is working correctly and I have verified it has the same issue on MS Edge on a Windows phone.

Oh okay! 

I haven't dig deep into the library! Thanks for helping out :)